The present invention relates to laser trimming of an accelerometer proof mass.
An accelerometer of the type described herein typically employs a damped proof mass (also known as seismic mass) supported by one or more hinges. Under the influence of external accelerations the proof mass deflects from its neutral position. This deflection is measured and nulled by application of an electromagnetic force feedback. The acceleration is calculated from the measured value of force feedback required to null the proof mass deflection.
Amorphous quartz is commonly used to fabricate the accelerometer proof mass. Quartz has two qualities that make it particularly attractive for proof mass use; 1) It has a very small coefficient of thermal expansion and; 2) it is brittle or in other terms its ductility is very low. The first of these properties results in high stability over a large temperature range (e.g. −40 C to +150 C). The second of these properties results in excellent shock performance. The proof mass and support hinges are not deformed by high shock.
The quartz proof mass is conventionally fabricated using wet-etch techniques. Hydrofluoric (HF) acid is typically used as an etchant which attacks quartz at a high etch rate. Essentially the proof mass is coated with a photoresist that is exposed in areas where the proof mass is to be etched. The etch solution is generally a buffered and diluted HF acid solution. The proof mass is immersed in the acid solution for a specific time to enable thinning down of the proof mass support hinges. Generally the etch process reduces the hinge thickness from 0.030 inch to about 0.001 inch. However, stopping the etch process at the optimum hinge thickness can be difficult because of the minor variations in the etch rate (typically 0.005″/hour at an etch solution temperature of 70° C.).